Wireless communications system that supports multiple modes of operation

ABSTRACT

A wireless communications adapts its mode of operation between spatial multiplexing and non-spatial multiplexing in response to transmission-specific variables. An embodiment of a wireless communications system for transmitting information between a base transceiver station and a subscriber unit includes mode determination logic. The mode determination logic is in communication with the base transceiver station and the subscriber unit. The mode determination logic determines, in response to a received signal, if a subscriber datastream should be transmitted between the base transceiver station and the subscriber unit utilizing spatial multiplexing or non-spatial multiplexing in an embodiment, the mode determination logic has an input for receiving a measure of a transmission characteristic related to the received signal. In an embodiment, the mode determination logic includes logic for comparing the measured transmission characteristic to a transmission characteristic threshold and for selecting one of spatial multiplexing and non-spatial multiplexing in response to the comparison of the measured transmission characteristic to the transmission characteristic threshold. In an embodiment, the transmission characteristic includes at least one of delay spread, post-processing signal-to-noise ratio, cyclical redundancy check (CRC) failure, residual inter-symbol interference, mean square error, coherence time, and path loss. By adapting the mode of operation in response to transmission-specific variables, the use of spatial multiplexing can be discontinued in unfavorable conditions. Additionally, because the wireless communications system can adapt its mode of operation between spatial multiplexing and non-spatial multiplexing, the communications system is compatible with both subscriber units that support spatial multiplexing and subscriber units that do not support spatial multiplexing.

FIELD OF THE INVENTION

The invention relates generally to wireless communications systems thatuse multiple access protocols. More particularly, the invention relatesto a wireless communications system that supports wireless transmissionwith spatial multiplexing and without spatial multiplexing.

BACKGROUND OF THE INVENTION

Wireless communications systems use multiple access protocols, such astime-division multiple access (TDMA), frequency-division multiple access(FDMA), code-division multiple access (CDMA), and space-divisionmultiple access (SDMA) protocols, to enable wireless communicationsbetween base transceiver stations and multiple subscriber units.Typically, a wireless communications system includes multiple basetransceiver stations that are spaced to create subscriber cells.Subscriber units, which may include mobile or fixed units, exchangeinformation between a nearby base transceiver station over a dedicatedradio frequency.

First generation wireless communications systems utilize single antennatransceivers to exchange information between a base transceiver stationand a subscriber unit. While wireless communications systems withsingle-antenna transceivers work well, they are subject to channeldegradation from, for example, multipath fading and/or signalinterference. Next generation wireless communications system have beendeveloped which utilize multiple spatially separated antennas at thebase transceiver station and/or the subscriber unit to transmit asubscriber datastream over multiple paths. Transmitting a subscriberdatastream between a base transceiver station and a subscriber unit overmultiple paths is referred to as transmission diversity. Because thetransmission antennas are spatially separated, the receiving devicereceives diverse signals that can be processed to reduce multipathfading and to suppress interfering signals.

While transmission diversity involves transmitting a subscriberdatastream over multiple paths, each of the transmitted datastreamscarries the same information. New wireless communications systems havebeen developed that utilize multiple spatially separated antennas at thebase transceiver stations and the subscriber units to multiplextransmissions in order to increase the bit rates between the basetransceiver stations and the subscriber units. The technique, known asspatial multiplexing, is described in U.S. Pat. No. 6,067,290, which isassigned to the assignee of the present invention.

Although spatial multiplexing works well to increase the bit ratebetween base transceiver stations and subscriber units that supportspatial multiplexing, not all base transceiver stations and subscriberunits support spatial multiplexing. In addition, even when a basetransceiver station and a subscriber unit both support spatialmultiplexing, it may not be appropriate to utilize spatial multiplexingfor every transmission. For example, transmission conditions may be suchthat spatial multiplexing provides poor results, thereby eliminating thepossibility of an increased bit rate that is provided by spatialmultiplexing.

In view of the fact that not all base transceiver stations andsubscriber units in wireless communications systems support spatialmultiplexing and in view of the fact that spatial multiplexing may notalways be the most effective transmission technique between basetransceiver stations and subscriber units even when both devices supportspatial multiplexing, what is needed is a wireless communications systemand method that adapts its mode of operation between spatialmultiplexing and non-spatial multiplexing in response to varioustransmission-specific variables.

SUMMARY OF THE INVENTION

A system and method for wireless communications adapts its mode ofoperation between spatial multiplexing and non-spatial multiplexing inresponses to transmission-specific variables. By adapting the mode ofoperation in response to transmission-specific variables, the use ofspatial multiplexing can be discontinued in unfavorable conditions.Additionally, because the wireless communications system can adapt itsmode of operation between spatial multiplexing and non-spatialmultiplexing, the communications system is compatible with bothsubscriber units that support spatial multiplexing and subscriber unitsthat do not support spatial multiplexing.

An embodiment of a wireless communications system for transmittinginformation between a base transceiver station and a subscriber unitincludes mode determination logic. The mode determination logic is incommunication with the base transceiver station and the subscriber unit.The mode determination logic determines, in response to a receivedsignal, if a subscriber datastream should be transmitted between thebase transceiver station and the subscriber unit utilizing spatialmultiplexing or non-spatial multiplexing.

In an embodiment, the mode determination logic has an input forreceiving a measure of a transmission characteristic related to thereceived signal. In an embodiment, the mode determination logic includeslogic for comparing the measured transmission characteristic to atransmission characteristic threshold and for selecting one of spatialmultiplexing and non-spatial multiplexing in response to the comparisonof the measured transmission characteristic to the transmissioncharacteristic threshold. In an embodiment, the transmissioncharacteristic includes at least one of delay spread, post-processingsignal-to-noise ratio, cyclical redundancy check (CRC) failure, residualinter-symbol interference, mean square error, coherence time, and pathloss.

In an embodiment, the base transceiver station and the subscriber unitutilize a multiple access protocol to transmit the subscriberdatastream, wherein the multiple access protocol is selected from atleast one of a group of multiple access protocols consisting of:code-division multiple access, frequency-division multiple access,time-division multiple access, space-division multiple access,orthogonal frequency division multiple access, wavelength divisionmultiple access, wavelet division multiple access, orthogonal divisionmultiple access, and quasi-orthogonal division multiple access.

In an embodiment, mode determination information is communicated betweenthe base transceiver station and the subscriber unit over a controlchannel that includes a dedicated mode identification field.

In an embodiment, the mode determination logic has an input forreceiving system specifications related to the signal receive capabilityof at least one of the base transceiver station and the subscriber unit.

In an embodiment, the mode determination logic has an input forreceiving system specifications related to the signal transmitcapability of at least one of the base transceiver station and thesubscriber unit.

In an embodiment, the mode determination logic includes logic fordetermining if both the base transceiver station and the subscriber unitsupport spatial multiplexing, and for selecting non-spatial multiplexingfor transmission of the subscriber datastream if one of the basetransceiver station and the subscriber unit does not support spatialmultiplexing.

In an embodiment, non-spatial multiplexing transmission modes includesingle-carrier transmissions, multi-carrier transmissions, multi-codetransmissions, transmit diversity, and beamforming. In an embodiment,transmit diversity includes selection diversity, space-time coding, andmaximum ratio combining. In an embodiment, the base transceiver stationand the subscriber unit are capable of transmitting the subscriberdatastream in more than one of said non-spatial multiplexingtransmission modes.

In an embodiment, the mode determination logic resides within thesubscriber unit and in another embodiment, the mode determination logicresides within the base transceiver station.

A method for operating a wireless communications system involvesreceiving a signal transmission at one of a base transceiver station anda subscriber unit via the wireless communications system, anddetermining, in response to the received signal transmission, whether asubscriber datastream should be transmitted between the base transceiverstation and the subscriber unit utilizing spatial multiplexing ornon-spatial multiplexing.

In an embodiment, the method includes additional steps of measuring atransmission characteristic of the received signal transmission,comparing the measured transmission characteristic to a transmissioncharacteristic threshold, and selecting one of spatial multiplexing andnon-spatial multiplexing in response to the comparison of the measuredtransmission characteristic to the transmission characteristicthreshold.

In an embodiment, the transmission characteristic includes at least oneof delay spread for the received signal transmission, post-processingsignal-to-noise ratio for the received signal transmission, CRC failurefor the received signal transmission, residual inter-symbol interferencefor the received signal transmission, mean square error for the receivedsignal transmission, coherence time of the channel, and path loss.

Another embodiment of a wireless communications system for transmittinginformation between a base transceiver station and a subscriber unitincludes a control channel for communicating if a subscriber datastreamshould be transmitted between the base transceiver station and thesubscriber unit utilizing spatial multiplexing or non-spatialmultiplexing.

In an embodiment, the control channel is used to indicate the spatialmultiplexing or non-spatial multiplexing capability of one of the basetransceiver station and the subscriber unit.

In an embodiment, the control channel is used to indicate a change inthe mode of operation between spatial multiplexing and non-spatialmultiplexing.

In an embodiment, the control channel includes a dedicated field in aframe that is part of the subscriber datastream.

In an embodiment, the wireless communications system further includesmode determination logic, in communication with the base transceiverstation and the subscriber unit, for determining, in response to areceived signal, if a subscriber datastream should be transmittedbetween the base transceiver station and the subscriber unit utilizingspatial multiplexing or non-spatial multiplexing.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a wireless communications system that includes two basetransceiver stations and two subscriber units that support spatialmultiplexing and a subscriber unit that does not support spatialmultiplexing.

FIG. 2 depicts a spatially multiplexed transmission between a basetransceiver station and subscriber unit.

FIGS. 3A-3C depict example transmissions between a base transceiverstation and a subscriber unit that represent how a wirelesscommunications system adapts its mode of operation between spatialmultiplexing and non-spatial multiplexing in response totransmission-specific variables in accordance with an embodiment of theinvention.

FIG. 3D depicts a stream of frames, with each frame including a payload,a header, and a control channel for communicating mode determinationlogic in accordance with an embodiment of the invention.

FIG. 4 depicts transmission-specific inputs to mode determination logicthat are considered to determine the appropriate mode of operationbetween spatial multiplexing and non-spatial multiplexing in accordancewith an embodiment of the invention.

FIG. 5 is a process flow diagram of a method for adapting the mode ofoperation of a wireless communications system between spatialmultiplexing and non-spatial multiplexing in response totransmission-specific variables in accordance with an embodiment of theinvention.

FIG. 6 is a process flow diagram of another method for adapting the modeof operation of a wireless communications system between spatialmultiplexing and non-spatial multiplexing in response totransmission-specific variables in accordance with an embodiment of theinvention.

FIG. 7 is a process flow diagram of a method for link initializationbetween a base transceiver station and a subscriber unit that adapts itsmode of operation between spatial multiplexing and non-spatialmultiplexing in response to transmission-specific variables inaccordance with an embodiment of the invention.

FIG. 8 depicts an embodiment of a base transceiver station and asubscriber unit that are equipped to adapt their mode of operationbetween spatial multiplexing and non-spatial multiplexing in response totransmission-specific variables in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in the figures for purposes of illustration, the invention isembodied in a system and method for wireless communications that adaptsits mode of operation between spatial multiplexing and non-spatialmultiplexing in response to transmission-specific variables. By adaptingthe mode of operation in response to transmission-specific variables,the use of spatial multiplexing can be discontinued in unfavorableconditions. Additionally, because the wireless communications system canadapt its mode of operation between spatial multiplexing and non-spatialmultiplexing, the communications system is compatible with bothsubscriber units that support spatial multiplexing and subscriber unitsthat do not support spatial multiplexing.

FIG. 1 depicts a multiple access wireless communications system thatincludes base transceiver stations (BTSs) 102 and 104 and subscriberunits (SUs) 106, 108, and 110. The base transceiver stations includespatially separated antennas 114 and spatial multiplexing logic 118. Thebase transceiver stations also include mode determination logic 122 thatenables the system to adapt its mode of operation between spatialmultiplexing and non-spatial multiplexing in response totransmission-specific variables. The mode determination logic is thefocus of the invention and is described in detail below. Multiple accessprotocols that may be utilized by the wireless communications systeminclude TDMA, FDMA, CDMA, SDMA, orthogonal frequency division multipleaccess (OFDMA), wavelength division multiple access (WDMA), waveletdivision multiple access, orthogonal division multiple access (ODMA),and quasi-ODMA protocols.

The antennas 114 of the base transceiver stations 102 and 104 aredefined to be spatially separate if they are capable oftransmitting/receiving spatially separated signals. A single antenna maybe used to transmit/receive spatially separate signals provided itincludes the ability to transmit/receive orthogonal radiation patterns.Hereinafter, the phrase “spatially separate” shall be understood toinclude any antenna, transmitter, or receiver capable of communicatingspatially separate signals. The spatial multiplexing logic 118 at thebase transceiver stations may work in conjunction with a master switchcenter (MSC) 126 to accomplish spatial multiplexing. Spatialmultiplexing is described in more detail below. The base transceiverstations are configured to communicate with subscriber units of atraditional type, i.e. those that do not support spatial multiplexing aswell as spatially enabled subscriber units, i.e. those that supporteither or both spatial multiplexing and non-spatial multiplexing.

Although the base transceiver stations 102 and 104 are shown as towers,the base transceiver stations may include satellites, balloons, planesetc. Additionally, the base transceiver stations may be located indoorsor outdoors. As is known in the field of wireless communications, basetransceiver stations are located to create coverage cells. Subscriberunits within a coverage cell exchange radio frequency (RF) signals withone or more base transceiver stations that make up the cell. Multiplecoverage cells combine to create a subscriber area.

The base transceiver stations 102 and 104 are connected to the masterswitch center 126 and the master switch center is connected to a network128 such as a circuit switched network or a packet switched network. Themaster switch center is the interface between the circuit/packetswitched network and the wireless communications system. The masterswitch center may include spatial multiplexing logic 130 that implementsa portion of the spatial multiplexing process and mode determinationlogic 132 that implements a portion of the mode adaptation process. Themaster switch center controls the switching between the circuit/packetswitched network and the base transceiver stations for allwireline-to-subscriber, subscriber-to-wireline, andsubscriber-to-subscriber communications. The master switch centerprocesses data received from the base transceiver stations that mayinclude, for example, subscriber unit status, diagnostic data, and billcompiling information. In an embodiment the master switch centercommunicates with the base transceiver stations using the X.25 protocolor IP protocol although other protocols are possible. The circuit/packetswitched network may include a public switched telephone network (PSTN)or other carrier network.

The subscriber units 106, 108, and 110 shown in FIG. 1 include devicesthat support spatial multiplexing and devices that do not supportspatial multiplexing. Subscriber units 108 and 110 support spatialmultiplexing and include two or more spatially separate antennas 134 andspatial multiplexing logic 136. Because the subscriber units supportspatial multiplexing, they also include mode determination logic 138,which may be used to determine the desired mode of operation for thesubscriber units in response to transmission-specific variables. Themode determination logic is the focus of the invention and is describedin more detail below. Subscriber unit 106 does not support spatialmultiplexing, does not include spatial multiplexing logic, and includesone or more antennas. Subscriber units that do not support spatialmultiplexing may or may not include mode determination logic. In theembodiment of FIG. 1, the subscriber unit 106 includes modedetermination logic 140 that notifies the base transceiver stations 102and 104 that the subscriber unit does not support spatial multiplexing.

The subscriber units 106, 108, and 110 may be mobile, portable, orfixed. Mobile subscriber units such as 106 and 108 may include mobiletelephones, personal digital assistants (PDAs), and automobile baseddigital assistants. Portable subscriber units may include, for example,laptop computers. Fixed subscriber units such as subscriber unit 110include customer premise equipment (CPE) that is coupled to a digitaldevice, or devices, such as a computer 142, a telephone 144, a computernetwork, and a television.

FIG. 2 is a graphical depiction of a base transceiver station 202 and asubscriber unit 208 that support spatial multiplexing. As shown in FIG.2, both the base transceiver station and the subscriber unit include aparser/combiner 210 and 212, a multiple access spatially configuredtransceiver 214 and 216, and at least two antennas 218 and 220. Theparser/combiner breaks a subscriber datastream into substreams andre-assembles substreams into the original subscriber datastream.Referring to FIG. 2, a subscriber datastream 224 enters the basetransceiver station 202 as a stream of sequential packets (or frames)b1, b2, b3, and b4 and is transmitted from the base transceiver stationto the subscriber unit 208. The parser 210 at the base transceiverstation divides the subscriber datastream into two substreams containingdifferent packets from the sequence of packets. For example, the parserdivides the subscriber datastream into one substream that includespackets b1 and b3 and into another substream that includes packets b2and b4.

The multiple access spatially configured transceivers 214 and 216perform various functions required for spatial multiplexing such as RFupconversion, RF downconversion, modulation, coding, decoding, andspatial processing. The spatially separate antennas 218 and 220 transmitand receive the spatially multiplexed signals. Referring to FIG. 2, thetwo substreams 226 and 228 at the base transceiver station 202 areprocessed by the multiple access spatially configured transceiver 214 ofthe base transceiver station and transmitted by the two spatiallyseparate antennas 218 to the subscriber unit 208. The signalstransmitted from the two base transceiver station antennas 218 aretransmitted over the same frequency and are received by the twosubscriber unit antennas 220. The multiple access spatially configuredtransceiver 216 at the subscriber unit 208 receives the two signals andutilizes spatial processing techniques to recover the two substreams 236and 238. The two recovered substreams are then re-assembled by thecombiner into the original subscriber datastream 234. The process isreversed when a subscriber datastream is transmitted from the subscriberunit to the base transceiver station.

It should be understood that FIGS. 1 and 2 are general depictions ofbase transceiver stations and subscribers units that support spatialmultiplexing and that spatial multiplexing can be accomplished usingmore than two antennas at the base transceiver station and/or thesubscriber unit. More detailed descriptions of spatial multiplexing arefound in U.S. Pat. Nos. 5,345,599 and 6,067,290, both of which areincorporated by reference herein.

Although spatial multiplexing from a single base transceiver station isdescribed with reference to FIG. 2, spatial multiplexing can also beaccomplished by transmitting a spatially multiplexed signal fromseparate base transceiver stations. That is, one substream, for examplethe b1/b3 substream 226, can be transmitted to the subscriber unit fromone base transceiver station while another substream, for example theb2/b4 substream 228, is transmitted to the same subscriber unit from aphysically separate base transceiver station. Referring back to FIG. 1,one substream can be transmitted from base transceiver station 102 whilethe other substream is transmitted from base transceiver station 104.The multiple base transceiver station implementation as opposed to thesingle base transceiver station implementation can provide informationsignals that are easier to separate. Separation is easier because themultiple base transceiver station antennas transmit information signalswhich are received at angles of arrival that are typically greater thanthe angles of arrival of signals transmitted by a single basetransceiver station with multiple antennas.

As described above, spatial multiplexing may not be supported by allbase transceiver stations and subscriber units. Subscriber datastreamsthat are not transmitted using spatial multiplexing are referred-to asnon-spatial multiplexed communications. Non-spatial multiplexedcommunications include any wireless communications techniques that donot utilize spatial multiplexing such as single input single outputcommunications and diversity communications. Single input single outputcommunications involve transmission of a subscriber datastream from asingle antenna transmitter to a single antenna receiver and are commonlysubject to multipath fading. Diversity communication is a technique usedin multiple antenna communications systems that reduces the effects ofmultipath fading. It may be applied to systems with multiple antennas atthe transmitter and/or receiver. Non-spatial multiplexed communicationsmay involve single-carrier transmissions, multi-carrier transmissions,multi-code transmissions, and beamforming.

Transmitter diversity can be obtained by providing a transmitter withtwo or more (N) antennas. The N antennas imply N channels that sufferfrom fading in a statistically independent manner. Therefore, when onechannel is fading due to the destructive effects of multipathinterference, another of the channels is unlikely to be simultaneouslysuffering from fading. By virtue of the redundancy provided by the Nindependent channels, a diversity receiver can often reduce thedetrimental effects of fading. As is known in the field of wirelesscommunications, transmitter diversity communications may includeselection diversity, space-time coding equal gain combining, and maximumratio combining.

Two types of diversity communication implementations are possible. Thefirst includes a single base transceiver station and the second includesmultiple base transceiver stations. In the single base transceiverstation implementation, the transmitter antennas that are used fortransmitting information signals are elements of an antenna array at asingle base transceiver station. In the multiple base transceiverstations implementation, transmitter antenna elements or antenna arraysare located at two or more base transceiver stations. The multiple basetransceiver stations implementation as opposed to the single basetransceiver station implementation can provide improved diversity gain.Diversity gain is improved because the multiple base transceiver stationantennas transmit information signals which are received at angles ofarrival that are typically greater than the angles of arrival of signalstransmitted by single base transceiver station antennas, and thereforeexperience highly independent fading characteristics. Although diversitycommunication may reduce the effects of multipath fading, diversity doesnot enable the increased bit rates that are enabled by spatialmultiplexing.

As stated above, the focus of the invention is a wireless communicationssystem that adapts its mode of operation between spatial multiplexingand non-spatial multiplexing in response to transmission-specificvariables. FIGS. 3A-3C depict example transmissions between a basetransceiver station 302 and a subscriber unit 308 that represent how thewireless communications system adapts its mode of operation betweenspatial multiplexing and non-spatial multiplexing in response totransmission-specific variables. Referring to FIG. 3A, the basetransceiver station transmits information 312 to the subscriber unitthat is utilized to determine the appropriate mode of operation betweenthe base transceiver station and the subscriber unit for subsequentsubscriber datastream transmissions. The subscriber unit responds backto the base transceiver station to transmit subsequent subscriberdatastreams using either spatial multiplexing or non-spatialmultiplexing. FIG. 3B represents a transmission 314 from the subscriberunit to the base transceiver station indicating spatial multiplexingshould be utilized for future transmissions. FIG. 3C represents atransmission 316 from the subscriber unit to the base transceiverstation indicating non-spatial multiplexing should be utilized forfuture transmissions. The transmission-specific variables that areconsidered to determine whether spatial multiplexing should be used forsubsequent transmissions are described in detail below.

In an embodiment, the base transceiver stations and subscriber unitscommunicate information related to the operation mode over a controlchannel. For example, a control channel may include a specified field ofbits in a transmitted bitstream that are dedicated to communicating modedetermination information. FIG. 3D represents the packets describedabove with reference to FIG. 2. Each of the packets (b1, b2, b3, and b4)includes a header 324 and a payload 322 as is known in the field ofpacket based networks. The packets also include a control channel 326,or field, which contains bits that are dedicated to communicatinginformation related to the mode of operation between the basetransceiver stations and the subscriber unit. The control channel may belocated in the header or the payload portion of each packet, however itis preferably located in the header. In another embodiment, a controlchannel may involve a dedicated transmission frequency a dedicated timeslot or some other identified communications channel. In an embodiment,the information 314 (shown in FIG. 3B), indicating that spatialmultiplexing is preferred, is transmitted to the base transceiverstation over the control channel and the information 316 (shown in FIG.3C), indicating that non-spatial multiplexing is preferred, istransmitted to the base transceiver station over the control channel. Inan embodiment, the information 312 (shown in FIG. 3A) that is used bythe subscriber unit for mode determination is not transmitted over thecontrol channel. For example the information 312 contains only thesubscriber datastream that is being transmitted from the basetransceiver station 302 to the subscriber unit 308. In anotherembodiment, the information that is used for mode determination includestraining information that is provided to establish and fine tune acommunications channel between a base transceiver station and asubscriber unit.

FIG. 4 represents transmission-specific inputs 450 and 452 that areconsidered by the mode determination logic 454 to determine theappropriate mode of operation for subsequent datastream transmissions.The mode determination logic may consider any one of, or combination of,transmission-specific variables in the mode determination process. Asshown in FIG. 4, the transmission-specific variables can be generallycategorized as transmission characteristics and system specifications.Although the identified variables are not meant to be exhaustive, thetransmission characteristics that may be considered in the modedetermination process relate to a characteristic of a received signaland include delay spread, post-processing signal-to-noise ratio,cyclical redundancy check (CRC) failures, residual inter-symbolinterference (ISI), mean square error, coherence time, and path loss.The transmission characteristics of a received signal will typicallyvary over time as transmission conditions change. The above-identifiedtransmission characteristics are described in more detail below.

The system specifications that may be considered in the modedetermination process include the transmitter type and/or the receivertype that are involved in a wireless communication. In particular, thereceiver type has a significant effect on the acceptable thresholds thatare established for certain transmission characteristics such as delayspread. The system specifications of wireless communications systems areset by the hardware and/or software capabilities of the base transceiverstation(s) and the subscriber units. Wireless communications systemsoften do not have uniform base transceiver station and subscriber unitcapabilities throughout the system and therefore, each base transceiverstation and subscriber unit combination has a unique set of hardwareand/or software capabilities. The capabilities of the base transceiverstation and subscriber unit combination can effect the desired mode ofoperation. Some system specifications that are considered in modedetermination are described in detail below.

In an embodiment, the mode determination logic 454 considers at leastone of the transmission characteristics 450 in the mode determinationprocess. In an embodiment, a transmission characteristic for a receivedsignal is measured and compared to a transmission characteristicthreshold. The mode of operation for a subsequent transmission isdetermined by whether or not the measured transmission characteristicexceeds the transmission characteristic threshold. Upon consideration ofone, or a combination, of the transmission-specific variables, the modedetermination logic outputs 456 an indication of the operation mode(either spatial multiplexing or non-spatial multiplexing) for asubsequent transmission. Because the mode determination logic determinesthe mode of operation in response to a measured transmissioncharacteristic, or characteristics, the system adapts to utilize a modeof operation that is preferred for the current set of conditions. In anembodiment, it is assumed that spatial multiplexing is the desired modeof operation and that the system defaults to non-spatial multiplexingwhen transmission-specific conditions are not favorable for spatialmultiplexing. In an embodiment, the transmission characteristicthreshold(s) vary over time and are effected by parameters such as thesystem specifications and the transmission conditions.

In an embodiment, a system specification 452 is considered in additionto a transmission characteristic 450 by the mode determination logic 454to determine the mode of operation for a subsequent transmission. Infact, the capability of the receiver involved in a wirelesscommunication is an important consideration in establishing thethreshold for a transmission characteristic. For example, a receiverwith a robust spatial processor will have a relatively high delay spreadthreshold.

Transmission Characteristics

Delay spread is a measure of the maximum amount of time over which asignal arrives at a receiver. Delay spread exists because a transmittedsignal propagates over multiple paths from the transmitter to thereceiver, with environmental conditions causing each path to have adifferent length and therefore transmission time. When the delay spreadis greater than the symbol period, the resulting channel will introduceinter-symbol interference ISI into the symbol stream.

In an embodiment, the measured delay spread for a received signal iscompared to a delay spread threshold and if the measured delay spreadexceeds the delay spread threshold, then the operation mode is switchedfrom spatial multiplexing to non-spatial multiplexing for subsequentdatastream transmissions. The delay spread threshold depends partiallyon the receiver type.

Receivers that attempt to remove the effects of a channel (i.e., linearreceivers, decision feedback receivers, and successive cancelingreceivers) deliver a symbol stream to a symbol block. Thesignal-to-noise ratio of the symbol stream is referred to as thepost-processing signal-to-noise ratio since it is computed afterremoving the effects of the channel and any other interferingtransmissions (in a vector transmission system). The post-processingsignal-to-noise ratio is often a function of the channel, the equalizerparameters, and the noise power. Larger post-processing signal-to-noiseratios are generally preferred to smaller post-processingsignal-to-noise ratios.

A post-processing signal-to-noise ratio threshold establishes theminimum acceptable signal-to-noise ratio required to ensure dataquality. The post-processing signal-to-noise ratio for a received signalis compared to the post-to processing signal-to-noise ratio thresholdand if the signal-to-noise ratio drops below the signal to noise ratiothreshold, then the operation mode is switched from spatial multiplexingto non-spatial multiplexing for subsequent datastream transmissions.

A CRC failure indicates an error in a transmission. A CRC failurethreshold can be established which ensures an acceptable transmissionquality when using spatial multiplexing. The number of CRC failures in aspatially multiplexed transmission between a base transceiver stationand a subscriber unit can be monitored and compared to the CRC failurethreshold. If the number of CRC failures exceeds the CRC failurethreshold, then the operation mode is switched from spatial multiplexingto non-spatial multiplexing.

Residual ISI is the ISI that remains after the receiver has processedthe received signal and has attempted to cancel the ISI. A largeresidual ISI indicates that all the ISI was not canceled. Residual ISIreduces the post-processing signal-to-noise ratio because it increasesthe effective noise and therefore increases the error rate. A residualISI threshold establishes the maximum acceptable residual ISI that isallowable to ensure data quality. The residual ISI for a received signalis compared to the residual ISI threshold and if the measured residualISI exceeds the residual ISI threshold, then the operation mode isswitched from spatial multiplexing to non-spatial multiplexing forsubsequent datastream transmissions. The residual ISI threshold dependspartially on the receiver type.

The mean square error indicates how well the filter coefficient ofminimum mean square error (MMSE), linear, and decision feedbackreceivers equalize a channel. A large mean square error indicates thatthere is a large amount of noise or that the channel is difficult toequalize. A mean square error threshold establishes the maximumacceptable mean square error allowable to ensure data quality. The meansquare error for a received signal is compared to the mean square errorthreshold and if the mean square error exceeds the mean square errorthreshold, then the operation mode is switched from spatial multiplexingto non-spatial multiplexing for subsequent datastream transmissions. Themean square error threshold depends partially on the receiver type.

The coherence time is a measure of how fast the channel is changing overtime. Essentially it is a statistical measure which reveals how the longthe channel remains effectively invariant. It is related to the Dopplerspread of the channel and the velocity of the subscriber unit. A smallcoherence time means the channel is changing rapidly while a largecoherence time means the channel is changing slowly. A coherence timethreshold establishes the maximum tolerable coherence time to ensuredata quality. The coherence time is measured for a received signal andis compared to the coherence time threshold. If the coherence timeexceeds the threshold then the operation mode is switched from spatialmultiplexing to non-spatial multiplexing for subsequent datatransmissions. The coherence time threshold partially depends on thereceiver type.

The path loss is a measure of signal attenuation from transmitter toreceiver. It is a function of the channel parameters and increases asdistance between transmitter and receiver increases. A path lossthreshold establishes the maximum tolerable path loss to ensure dataquality. The path loss is measured for a received signal and is comparedto the path loss threshold. If the coherence time exceeds the thresholdthen the operation mode is switched from spatial multiplexing tonon-spatial multiplexing for subsequent data transmissions. The pathloss threshold partially depends on the receiver type.

System Specifications

The type of receiver involved in a wireless communication is a keysystem specification that influences the transmission characteristicthresholds that are described above. Receivers that support spatialmultiplexing include maximum likelihood, multiple in multiple out (MIMO)decision feedback, MIMO linear equalization, and successive cancellationreceivers. Receivers that support non-spatial multiplexing includemaximum likelihood, multiple in multiple out (MIMO) decision feedback,MIMO linear equalization, and successive cancellation receivers.Although these receiver types are well known in the field of wirelesscommunications systems, a brief description of these receiver types isprovided at the end of the specification for background.

In an embodiment, the mode determination logic is located within thesubscriber units. In another embodiment, the mode determination logic islocated within the base transceiver stations and in another embodiment,the mode determination logic is distributed throughout the wirelesscommunication system, with some portions of the mode determination logicin the subscriber units, the base transceiver stations, and/or themaster switch center.

Systems necessary to measure, generate, and/or store the transmissioncharacteristics and the system specifications that are considered by themode determination logic may be located in the subscriber units, thebase transceiver stations, and/or the master switch center. In anembodiment, the above-described transmission characteristics areprocessed through a channel analyzer and the system specifications aremaintained in a system database. Both the channel analyzer and thesystem database are depicted in FIG. 8.

FIG. 5 is a process flow diagram of a method for adapting the mode ofoperation of a wireless communications system between spatialmultiplexing and non-spatial multiplexing in response totransmission-specific variables. In a step 502, a burst of data istransmitted. In a step 504, the burst of data is received. In a step506, a transmission characteristic of the burst of data is measured. Forexample, the measured transmission characteristic may include one ormore of delay spread, post-processing signal-to-noise ratio, CRCfailure, residual inter-symbol interference, mean square error,coherence time, path loss. In a step 508, the mode of operation isdetermined between spatial multiplexing or non-spatial multiplexing inresponse to the measured transmission characteristic. In an embodiment,the mode determination involves comparing a measured transmissioncharacteristic to a transmission characteristic threshold and selectingthe mode of operation in response to the comparison. In a step 510, themode of operation for subsequent transmissions is indicted and then theprocess is repeated. In an embodiment, the preferred mode of operationis indicated to the base transceiver station over a control channel. Asdescribed above, the control channel includes a specified field in apacket (or frame) for indicating the preferred mode of operation.

FIG. 6 is a process flow diagram of another method for adapting the modeof operation of a wireless communications system between spatialmultiplexing and non-spatial multiplexing in response totransmission-specific variables. The method of FIG. 6 differs from themethod of FIG. 5 in that only a change in the mode of operation isindicated for subsequent transmissions. That is, if the mode ofoperation is not going to change in response to a measured transmissioncharacteristic then the operation mode for subsequent transmissions isnot indicated to the respective transmitter. Referring to FIG. 6, in astep 602, a burst of data is transmitted. In a step 604, the burst ofdata is received. In a step 606, a transmission characteristic of theburst of data is measured. For example, the measured transmissioncharacteristic may include one or more of delay spread, post-processingsignal-to-noise ratio, CRC failure, residual inter-symbol interference,mean square error, coherence time, path loss. In a step 608, the mode ofoperation is determined between spatial multiplexing or non-spatialmultiplexing in response to the measured transmission characteristic. Atdecision point 610, if the mode of operation is going to be changed fromthe current mode, then at step 612, the change in the mode of operationfor subsequent transmissions is indicated and then the process isrepeated. As described above, a control channel can be used to indicatea change in the mode of operation. If the mode of operation is not goingto be changed from the current mode, then the next burst of data can bereceived at step 604 and then the process is repeated.

While the process flow diagrams of FIGS. 5 and 6 apply particularly tomaintaining a link between a base transceiver station, or basetransceiver stations, and a subscriber unit, the process flow diagram ofFIG. 7 can be applied to link initialization between a base transceiverstation(s) and a subscriber unit. For example, the link initializationmethod applies when a subscriber unit is powered up or when a subscriberunit enters a subscriber area (or cell). In a step 702 a subscriber unitis powered up or arrives in the subscriber area. In a step 704, thesubscriber unit goes through a preliminary initialization as is known inthe field of multiple access wireless communications. In an embodiment,the preliminary initialization may involve the subscriber unit sending acontrol signal (over a dedicated control channel) to the basetransceiver stations in the cell that indicates the capability of thesubscriber unit (for example, whether or not the subscriber unitsupports spatial multiplexing). At decision point 706, the subscriberunit receives a control signal 708 from a base transceiver station(s)that indicates whether or not the base transceiver station(s) supportsspatial multiplexing. In an embodiment, spatial multiplexing capabilityis indicated by a control signal that is transmitted over a dedicatedcontrol channel and the lack of spatial multiplexing capability isindicated by the lack of a control signal.

Based on the control signals and the capabilities of the basetransceiver station(s) and the subscriber unit, an initial decision ismade as to whether or not spatial multiplexing can be used fortransmission of subscriber datastreams. If the subscriber unit or thebase transceiver station(s) does not support spatial multiplexing, thenat step 710, the decision to use a non-spatial multiplexing mode ofoperation is made and communicated over the control channel. If both thesubscriber unit and the base transceiver station(s) support spatialmultiplexing, then at step 712, the mode of operation is determinedbetween spatial multiplexing and non-spatial multiplexing in response tothe transmission-specific variables described above with reference toFIG. 4. At decision point 714, if the mode of operation is determined tobe non-spatial multiplexing, then at step 710, a non-spatialmultiplexing mode of operation is used for subsequent datastreamtransmissions. If the mode of operation is determined to be spatialmultiplexing, then at step 716 the spatial multiplexing mode ofoperation is used for subsequent datastream transmissions. The method ofFIG. 7 creates a wireless communications system in which subscriberunits that do and do not support spatial multiplexing can be used withthe same infrastructure.

The methods of FIGS. 5-7 are also applicable to mode determinations thatare made in response to the location of a subscriber unit within a cellarray. That is, the mode of operation may be influenced by the locationof a subscriber unit within a cell or influenced by the subscriber unittraveling between cells. In some situations, one cell may supportspatial multiplexing while an adjacent cell does not support spatialmultiplexing. As the subscriber unit travels between cells, the mode ofoperation may change in response to the capabilities of the basetransceiver stations in the cell, the capability of the subscriber unit,and/or the proximity of the subscriber unit within the cell. Forexample, as a subscriber unit travels from a spatial multiplexingsupported cell to a non-spatial multiplexing cell, a subscriber unitusing spatial multiplexing must switch to a non-spatial multiplexingmode of operation. Alternatively, as a subscriber unit travels from anon-spatial multiplexing supported cell to a spatial multiplexingsupported cell, the subscriber unit may switch from a non-spatialmultiplexing mode of operation to a spatial multiplexing mode ofoperation. Handoff indicators that may be considered by the modedetermination logic include received signal strength from current andpotential base transceiver stations, interference measurements, andmeasurements of the signal-to-interference level between current anddesired base stations.

FIG. 8 depicts an embodiment of a base transceiver station 802 and asubscriber unit 808 that are equipped to adapt their mode of operationbetween spatial multiplexing and non-spatial multiplexing in response totransmission-specific variables. As shown in FIG. 8, the basetransceiver station includes a mode switch 810, a spatial multiplexingunit 812, a space-time modulation and coding unit 814, a non-spatialmultiplexing unit 816, an optional transmission diversity unit 818, RFupconversion units 820, and transmitter antennas 822. The subscriberunit includes receiver antennas 826, RF downconversion units 828, aspatial multiplexing receiver 830, a non-spatial multiplexing receiver832, a channel analyzer 834, a system database 836, and modedetermination logic 854. Although the base transceiver station andsubscriber unit are depicted with two antennas each, either or both ofthe base transceiver stations and subscriber units may include more thantwo antennas.

In operation, a signal is transmitted from the base transceiver station802 to the subscriber unit 808. In an embodiment, the transmitted signalis part of a subscriber datastream although this is not critical. Inanother embodiment, the transmitted signal is a training signal, part ofa subscriber data stream or sent over a control channel, which isutilized to calibrate the receiver electronics. If the signal istransmitted using the spatial multiplexing unit 812 of the basetransceiver station then the signal is processed by the spatialmultiplexing receiver 830 of the subscriber unit and if the signal istransmitted using non-spatial multiplexing unit 816 of the basetransceiver station then the signal is processed by the non-spatialmultiplexing receiver 832 of the subscriber unit. Whether the signal istransmitted using spatial multiplexing or non-spatial multiplexing, thereceived signal is analyzed by the channel analyzer 834 to measure atransmission characteristic such as delay spread, post-processingsignal-to-noise ratio, CRC failures, residual ISI, or mean square error,coherence time, or path loss. The system database 836 stores systemspecifications that can also be considered for mode determination. In anembodiment, the system database includes fixed parameters, such as thetype, or types, of receiver that is available in the subscriber unit forsignal processing

The mode determination logic 854 considers at least one of the measuredtransmission characteristics from the channel analyzer 834 andoptionally information from the system database 836 to determine whetherthe next mode of operation should be spatial multiplexing or non-spatialmultiplexing. Determining the mode of operation preferably involvescomparing a measured transmission characteristic to a transmissioncharacteristic threshold as described above. A signal indicating themode determination from the mode determination logic is transmitted overa control channel to the mode switch 810 as represented by feedback loop860. The mode switch is set to either the spatial multiplexing mode ornon-spatial multiplexing mode in response to the signal from the modedetermination logic. The setting of the mode switch dictates the mode ofoperation for subsequent subscriber datastream transmissions. Theprocess of considering transmission-specific variables and adapting themode of operation in response to the transmission-specific variables isrepeated at a cycle time that can be adjusted as necessary. The modedetermination process can be reversed when a subscriber datastream istransmitted from the subscriber unit to the base transceiver station.

Brief Description of Receivers

This section describes various receiver algorithms known in the field ofwireless communications. The goal of a receiver is to provide anestimate of the transmitted symbol stream. For the purpose of discussionwe suppose that symbols s(0), . . . , s(N) are transmitted through achannel h_(c)(t), t=[0, t_(max)] and received as y(n)=h(n)*s(n)+v(n)where * is the convolution and h(n) is means h_(c)(nTs), the channelsampled at the symbol period.

Maximum Likelihood (Vector-MLSE) Receiver—This is the optimum receiverwhen the transmitted sequences are equally likely. The maximumlikelihood receiver estimates the transmitted sequence as the sequenceof symbols which maximizes the likelihood (in a mathematical sense).Formally, the maximum likelihood estimate is the symbol stream s(0), . .. , s(N) such that Probability(observe y(0), . . . , y(N)|given s(0), .. . , s(N)) is larger than all other symbol streams. Since s(n) comefrom a finite alphabet, there are a finite number of sequences overwhich to search. This receiver is typically implemented using theViterbi algorithm.

Linear Equalization Receiver—Suppose the channel impulse response ish(n), nonzero for n=0, . . . N. The goal of linear equalization is tofind a filter g(n), nonzero for n=0, . . . K, such that g(n)*h(n) (theconvolution of g(n) and h(n)) satisfies a certain criterion. Potentialequalization criteria include zero forcing and minimum mean squarederror (MMSE). The zero-forcing criterion is to design g(n) such that|g(n)*h(n)−d(n−D)|̂2 is as small as possible (where d(n) is the deltafunction and D is a chosen delay). The MMSE criterion is to design g(n)such that E[g(n) y(n)−s(n)]̂2 is minimized. This leads to the well knownWiener equalizer. In an embodiment, a linear equalizer performs thefollowing steps. (1) estimate channel h(n), noise variance, etc.; (2)form estimate equalizer g(n); (3) filter the received data, i.e., forms(n)=g(n)*y(n); and (4) estimate the transmitted symbols from s(n).

Decision Feedback Receiver—Unlike the maximum likelihood receiver, thelinear equalization receiver described above tries to remove the effectsof the channel irrespective of the finite alphabet property of s(n). Oneapproach to improve the performance of the linear receiver is to includea feedback loop that operates on the post-detection symbols. Aftersampling, the received signal y(n) passes through a feedforward filterf(n). The filter is designed to remove the pre-cursor ISI from thechannel impulse response h(t), the ISI due to future symbols. Pastsymbols are passed through the feedback filter b(n) and subtracted fromthe input sequence. Filter coefficients for f(n) and b(n) can be foundagain using MMSE or zero-forcing criteria although the formulae are moreinvolved.

Successive Cancellation Receiver—The maximum likelihood, linearequalization, and decision feedback receivers can be used in eitherscalar (systems with one transmit antenna) or vectors (systems withmultiple antennas). A successive cancellation receiver is suitable onlywhen there are multiple transmissions. Suppose a signal s1(n) istransmitted from antenna 1 and a signal s2(n) is transmitted fromantenna 2. These symbols are mixed by the channel as they arrive at thereceiver. To estimate the symbol streams, symbol stream s1(n) isestimated assuming that s2(n) is noise using one of the above receivers.Then the contribution of s1(n) is subtracted out and s2(n) is estimatedin a likewise fashion. A good successive cancellation receiver will havean algorithm that chooses the better of s1(n) or s2(n) for the firststep of decoding.

What is claimed is:
 1. A wireless communications system for transmittinginformation between a base transceiver station and a subscriber unitcomprising: mode determination logic, in communication with said basetransceiver station and said subscriber unit, for determining inresponse to a received signal, if a subscriber datastream should betransmitted between said base transceiver station and said subscriberunit utilizing spatial multiplexing or non-spatial multiplexing.
 2. Thewireless communications system of claim 1 wherein said modedetermination logic has an input for receiving a measure of atransmission characteristic related to said received signal.
 3. Thewireless communications system of claim 2 wherein said modedetermination logic includes logic for: comparing said measuredtransmission characteristic to a transmission characteristic threshold;and selecting one of spatial multiplexing and non-spatial multiplexingin response to said comparison of said measured transmissioncharacteristic to said transmission characteristic threshold.
 4. Thewireless communications system of claim 3 wherein said transmissioncharacteristic threshold varies in response to system conditions.